This invention relates to the manufacture of composite materials. More particularly, the invention relates to manufacture of composites made of reinforcing fibers imbedded in a curable resin.
Traditionally, fiber bundles have been impregnated with a selected resin by drawing bundles of the fiber through an open bath containing the resin in solution and then subjecting the wetted fiber bundles to a curing process. However, there are several difficulties associated with an open bath type impregnation process. These difficulties include the formation of voids when air is trapped between the resin and the fibers, encapsulation rather than impregnation of the fibers by the resin, entrainment of air in the resin solution, the hazardous conditions created by the use of highly volatile solvents, slow fiber bundle take-off speeds and the lack of control over the fiber to resin content of the product.
Alternatively, resin impregnation can be accomplished in an enclosed chamber to contain the volatile solvent compounds and to prevent air from being entrained in the resin bath solution. However, the use of an enclosed chamber does not prevent the formation of voids that detract from the quality of the composite product. Further, the use of an enclosed bath alone does not provide any additional control over the ratio of fiber to resin in the final product. Additional developments that have been intended to overcome these latter difficulties include the use of rollers and tapered openings to compress the resin into the fiber bundles. Further, enclosed bath type systems have been modified to be operated under reduced pressure to facilitate the removal of air that might otherwise be entrapped and create voids in the cured product.
The prior art has attempted to remove encapsulated air pockets from the impregnated fiber bundles through the manipulation of the pressure on those bundles and various mechanical means for pressing the resin into the bundles. As noted above, several attempts have been made to impregnate the fiber bundles under vacuum so that less air is available for creating voids. Further rollers have been used to press excess resin into the fiber bundles.
Japanese Patent Application Number 51039769, entitled xe2x80x9cFiber Reinforced Plastic Mouldingsxe2x80x9d and Japanese Patent Specification Number 52117966, entitled xe2x80x9cFiber Bundles Continuous Impregnation with Resinxe2x80x9d both disclose the use of a enclosed resin bath that is operated under reduced pressure. The fiber bundles are drawn into the enclosure and redirected through the resin bath by guide rollers. These disclosures emphasize the use of rollers for pressing excess resin into the fiber bundles. The impregnated fiber bundles are then drawn out of the enclosure through a thin slot which maintains the vacuum and removes excess resin. A primary difficulty with these types of systems is that the physical manipulation of the fiber bundles during impregnation, whether through the use of guide rollers or contact between the fiber bundles and the walls of a narrowed opening, causes breakage and or fraying of the fibers. Broken and frayed fibers are to be avoided since they will foul and clog an impregnation device requiring significant operational down time during cleaning and system preparation. In addition, damaged fiber bundles will not be uniform and will produce a poor quality composite material. Therefore, it is preferred to minimize, and if possible to eliminate, contact with or manipulation of the fiber bundles during their impregnation.
Another prior art device which uses reduced pressure to prevent or remove air pockets from the impregnated resin is disclosed in Japanese Patent Application Number [Kokai] 2-208021, entitled xe2x80x9cMethod of Manufacturing Fiber-Reinforced Plastic Forming Materials.xe2x80x9d A view of the device of this application is shown in FIG. 1. This application describes the use of multiple chambers in which the fiber bundles are in sequence subjected to heating, vacuum, a tapered slit and resin impregnation of the top half of the fiber bundles followed by repeated applications of vacuum, a tapered slit and resin impregnation of the bottom half of the bundles. The tapered openings have increasingly larger cross sectional areas as the impregnated fiber bundles pass downstream. The final tapered opening is used to determine the final amount of resin on the fiber bundles. It is claimed that perfect impregnation can be achieved by the method of impregnating the top and bottom halves of the fiber bundles separately and applying a pressure reduction treatment after each impregnation. However, it has been found that the use of reduced pressure treatments is less than satisfactory at achieving void-free resin impregnated fiber bundles, particularly at high fiber take-off rates. Rather, it has been discovered through the present invention that it is preferable to use positive pressure prior to and during resin impregnation to prevent and eliminate voids in the fiber bundles.
German Patent Application No. DE 2 824 376, entitled xe2x80x9cMethod of Impregnating Fibers,xe2x80x9d discloses an enclosed resin bath. As shown in FIG. 3, degassed resin is pumped into the bath from below the fiber bundles. The fiber bundles are directed into and through the resin on guide rolls. Further, the fiber bundles are drawn through three downstream-tapering conical dies that compress the impregnated resin and encapsulated air. According to the specification of DE 2 824 376, encapsulated air xe2x80x9cexplosivelyxe2x80x9d escapes as the fiber bundles exit the conical dies ripping the fiber bundles apart. Resin is pumped from the top of the bath and filtered to remove the air and free fibers before re-injection at inlets located upstream and below each of the conical dies. The impregnated fiber bundles are passed through a stripping die as they exit the resin bath housing.
Although DE 2 824 376 is directed at eliminating air pockets from the impregnated fiber bundles it does not address or attempt to prevent air from becoming entrapped. The use of guide rollers and stripping dies result in unnecessary contact with the impregnated fiber bundles that will damage the integrity of the bundles. Further, the use of the tapered structures to cause an xe2x80x9cexplosivexe2x80x9d release of entrapped air will likewise cause damage to the bundles that will foul the stripping die and contaminate the resin bath.
Not unlike DE 2 824 376, German Patent Application No. D 4 121 200, entitled, xe2x80x9cMethod of Impregnating Fibersxe2x80x9d describes using tapered slots to create an initial pressure on the fiber bundles that is quickly released in order to release entrapped air. Shown in FIG. 2, DE 4 121 200 discloses a channel with a succession of tapered dies. The specification of DE 4 121 200 emphasizes that excessive pressure on the fiber bundles should be avoided to prevent breaking or otherwise damaging the fibers. Resin outlets between the dies provide means of releasing excess pressure by drawing excess aerated resin and loose fibers out of the channel. The resin and loose fibers are filtered and re-injected prior to the first tapered die. The specification asserts that air entrained with the resin escapes from the channel at the fiber inlet as the resin is re-injected into the channel. The last tapered die strips excess resin to control the final amount of resin that remains on the fiber bundles.
The disclosure of DE 4 121 200 does not suggest any means for preventing air from being encapsulated in the resin initially. To the contrary, it is doubtful that all of the air entrained in the recirculating resin will be given off at the fiber inlet as is suggested and that therefore, the device will re-inject aerated resin into the channel to be re-encapsulated with the fiber bundles. Secondly, the reliance on physical contact with the impregnated fiber bundles such as with a stripping die will have undesirable results on the integrity of the fiber bundles and the reliability of the impregnation apparatus.
European Patent Specification EP 0 542 709, entitled xe2x80x9cMethod and Apparatus For Making Composite Materials,xe2x80x9d discloses preheating the fiber bundles, separating them using a floating mandrel, injecting resin around the separated fibers and then passing the fibers and resin through a tapered section to create a pressure head on the impregnated fibers. The reference claims that greater wetout and impregnation are achieved due to the pressure head, the heating of the fibers and the capillary action of the fibers. Further, although the reference emphasizes the creation of a pressure head on the fibers, it also asserts that resin injection should be controlled so that excess resin does not flow upstream from the impregnation chamber. In fact, the application claims that a seal should be located at the entrance to the impregnation chamber to prevent a back flow of resin out of the impregnation chamber.
EP 0 542 709 does not address the prevention or elimination of air voids but rather is directed at obtaining faster resin impregnation of the fibers. Although faster take-off speeds may be achieved by separating and manipulating the fibers with a floating mandrel and compacting them prior to impregnation, such handling of the fibers is unnecessary and is accompanied by excessive damage and fraying of the fiber bundles.
It has been found that voids can be reduced or eliminated without the use of complex vacuum systems and their interfering seals by using high pressure treatments such as with the use of rollers that can compress excess resin into the fiber bundles. Tapered structures and venturi-like structures have also been used in the art to compress the resin and air into the fiber bundles before abruptly releasing that pressure so that encapsulated air escapes the resin and fiber bundles violently, thereby removing the air and separating the fiber for impregnation. Nevertheless, as has been noted above, mechanical means such as the use of guide rollers and stripping dies that manipulate the fiber bundles during their impregnation and the use of structures designed to cause a violent release of air from the fiber bundles, should be avoided in order to maintain the integrity of the fiber bundles and the reliability of the impregnation apparatus. It is also evident that although these developments are directed at eliminating encapsulated air from resin impregnated fiber bundles, there does not appear to have been any development directed at preventing the air from being encapsulated between the fiber bundles and resin.
In some aspects, the invention relates to an apparatus for impregnating fiber bundles with resin which includes a resin impregnation head. The impregnation head comprises an upstream, tapered chamber which receives a fiber bundle through a fiber inlet and conveys it to an orifice. The orifice is rectangular with an adjustable cross sectional area where the fiber bundle fills 45%-70% of the area. The fiber bundle is pulled through the orifice to a downstream, tapered chamber and conveyed to a fiber outlet. The fiber outlet with an adjustable cross sectional area which is equal to the cross sectional area of the orifice. Two resin conduits are located immediately downstream of the orifice which direct a resin flow at an acute angle in relation to the fiber bundle.
In an alternative embodiment, the resin impregnation head further comprises tapered chambers which receive a fiber bundle wound around a mandrel, through a fiber inlet and conveys it to an orifice on through to a fiber outlet. The rectangular orifice has an adjustable cross sectional area of where the fiber bundle and mandrel fill 45%-70% of the cross sectional area. An inflatable, resilient member is located between the upstream and downstream chamber which does not physically contact the fiber bundle. Another inflatable, resilient member is located at the fiber outlet which does not physically contact the fiber bundle.
In an alternative embodiment, the invention relates to a method for impregnating fiber bundles with resin which comprises injecting a resin into a downstream chamber of a resin impregnation head comprising a fiber inlet, an upstream chamber, an orifice, a downstream chamber, and a fiber outlet. Next, an equilibrium of resin flow is established which minimizes the resin seepage from the fiber inlet and fiber outlet, such that the resin fills the downstream chamber and forms a resin wall within a partially filled upstream chamber. The equilibrium is established by adjusting the velocity of the fiber bundles to 0.5-20.0 ft./min., the viscosity of the resin to 100-1000 centipoise, the pressure of the resin injection to 10-150 psi, and the ratio of the cross sectional area of the orifice and the cross sectional area of the fiber outlet to 1:1. Finally, a fiber bundle is drawn into the fiber inlet through the upstream chamber and the resin wall to the orifice, and drawing the fiber bundle through the orifice into the downstream chamber, and out of the fiber outlet.
In an alternative embodiment, the method for impregnating fiber bundles with resin further comprises drawing a fiber bundle wound around a mandrel into the fiber inlet through the upstream chamber and the resin wall to the orifice and through the orifice into the downstream chamber, and out of the fiber outlet.
Advantages of the invention may include one or more of the following: a resin impregnation device and method that utilize high pressure to prevent and eliminate air pockets and voids in the resin impregnated fiber bundles and to achieve a more complete wet-out of continuous fiber bundles. It is essential however, that these high pressure treatments be accomplished with minimal manipulation or contact with the fiber bundles so as to achieve high speed impregnation and high uniformity in the product while preserving the integrity of the fiber bundles and the reliability of the impregnation apparatus.
The present invention uses a resin filed impregnation chamber that is free of air and other gases and that is maintained under positive pressure. The pressure within the impregnation chamber is sufficient to create a back flow of resin that is directed upstream against the movement of the fiber bundles. A xe2x80x9cwallxe2x80x9d of resin is maintained upstream from the impregnation chamber to minimize the air that is trapped between the resin and the fiber bundles and to prevent air from entering the high pressure impregnation chamber.